Forms of EDTA...? - (May/19/2010 )

We usually bought the disodium EDTA-2H2O. I think that is listed in Sambrook as well for making stock solutions. You can buy EDTA (solid form) in about six different salts or something like that, with various salts of sodium and in various hydration states. I have heard that for example, EDTA free acid is much less soluble than one of the sodium salts. Maybe this is a stupid question, but which form is best to buy? Is it more soluble the more sodium ions it is with or more hydrated or is there a limit? You have to add much NaOH to dissolve it anyway, no matter what right? Is there a price difference per number of moles of each? Just curious.

Thanks.

Usually you want the disodium salt, and then plan on adding equimolar amounts of NaOH (roughly) to dissolve it and bring it to pH 8.0, which is roughly speaking the trisodium salt. Or you can buy the pre-made 0.5 M solution, which avoids the problem of making it. Since the amount used is usually so small, this may be an effective thing to do.